Unwanted vibrations are generally present in a grinding machine, and are denerated by out of balance conditions of the grinding wheel due to various possible reasons such as: shape and/or constitution defects (parallelism errors between the faces of the wheel, concentricity errors between the external abrading surface and the internal centering hole, inhomogeneity of the material, porosities, etc.), inaccurate assembling to the rotating spindle (causing the wheel center of gravity being spaced apart from the rotation axis), and, in general, deteriorations due to wear and/or splinter during the machining operations of the workpieces. The vibrations may cause inaccuracies in the machined workpieces such as roundness errors (ovality, lobing), and introduce loads and stresses that may damage the machine tool.
Known balancing apparatuses are coupled to the grinding wheel and comprise movable masses, that are driven by electrical motors, during the rotation of the wheel, along radial or angular paths, in order to compensate the out of balance.
The driving motors are part of the apparatus, rotate along with it and along with the wheel, and are fed by a fixed external power source, through an electrical coupling including, for example, slip rings and brushes.
The vibration caused by the out of balance is picked up by a sensor, and some parameters of the vibration (frequency, amplitude, and/or other ones) are valued, and then are displayed and/or processed in a unit that also comprises the above mentioned power source. The unit may then send proper "balancing" signals to the motors, in order to properly drive the movable masses.
A balancing apparatus comprising the above mentioned features is shown and described in patent U.S. Pat. No. 3698263. According to such patent, the driving motors are controlled by an operator on the basis of the vibration parameters that are displayed in an instrument.
A problem of the known apparatuses is represented by the wear of the brushes that remain in contact with the relevant slip rings in the course of the grinding wheel rotation, and by the consequent need of relatively frequent maintenance and replacement operations.
DE-A-4104350 discloses machine tools comprising a machining head carrying a tool, a piezoelectic transducer rotating with the tool, a control unit for generating electrical signals for driving the piezoelectic transducer, and an actuating device including slip rings and brushes for electrically connecting the piezoelectic transducer with the control unit.
The actuating device includes a pneumatic cylinder having an axially movable piston rod coupled to a brush holder. Springs may be interposed between the brushes and the brush holder for pushing the brushes onto the slip rings.
The machine tool can operate according to two machining modes, one of which is an ultrasonic mode in which the actuating device brings the brushes into contact with the slip rings and the tool is ultrasonically vibrated by the piezoelectic transducer.
The pneumatic cylinder may be replaced, in a manner not described, with an actuator such as a motor or a solenoid, as a means to move the brushes.
In the machine tools shown in the German patent application the brushes are urged both in contact with the slip rings and to a retracted position by means of the positive action of an unique actuating device, that, as a consequence, undergoes frequent operations and must include suitable driving and guiding mechanisms to drive in both senses the brush holder. Moreover, in case of a failure of the unique actuating device, the position of the brushes is not known for a certainty, and unproper working of the machine tool is possible.